Al-Mg-Si ALUMININUM ALLOY EXTRUDED PRODUCT EXHIBITING EXCELLENT FATIGUE STRENGTH AND IMPACT FRACTURE RESISTANCE

ABSTRACT

An aluminum alloy extruded product includes 0.3 to 0.8 mass % of Mg, 0.5 to 1.2 mass % of Si, 0.3 mass % or more of excess Si with respect to the Mg 2 Si stoichiometric composition, 0.05 to 0.4 mass % of Cu, 0.2 to 0.4 mass % of Mn, 0.1 to 0.3 mass % of Cr, 0.2 mass % or less of Fe, 0.2 mass % or less of Zr, and 0.005 to 0.1 mass % of Ti, with the balance being aluminum and unavoidable impurities, the aluminum alloy extruded product having a fatigue strength of 140 MPa or more, a fatigue ratio of 0.45 or more, and an interval between striations on a fatigue fracture surface of 5.0 μm or less.

Japanese Patent Application No. 2008-213384 filed on Aug. 21, 2008 andJapanese Patent Application No. 2009-135607 filed on Jun. 5, 2009, arehereby incorporated by reference in their entirety.

BACKGROUND

The present invention relates to an Al—Mg—Si aluminum alloy extrudedproduct that exhibits high fatigue strength, excellent impact fractureresistance, and excellent formability.

In recent years, automotive components made of aluminum have beenstudied and used in practice in order to reduce the weight ofautomobiles to improve travel performance and reduce fuel consumptionfrom the viewpoint of environment protection.

Since an aluminum alloy structural material used for automobiles or thelike is repeatedly subjected to impact during travel, it is necessary todesign the material taking account of the fatigue strength of thematerial.

Therefore, a high-strength material is used to provide fatigue strength.A component that is directly subjected to and absorbs impact duringtravel is also required to exhibit high impact fracture resistance.

However, high-strength aluminum alloys that have been proposed exhibitpoor extrusion productivity so that the production cost increases.

When producing an aluminum structural material used for automotiveunderbody parts or the like, the product may require press working orbending depending on the shape of the product. When using ahigh-strength material, cracks or orange peeling occur on the surface ofthe material during press working or bending. The fatigue strength ofthe material decreases due to such surface defects. Therefore, thesurface defects must be removed by a mechanical polishing step (e.g.,buffing) so that the production cost increases.

JP-A-2005-82816 discloses an aluminum alloy forged material thatexhibits high-temperature fatigue strength. However, the Al—Cu aluminumalloy disclosed in JP-A-2005-82816 is suitable for a forged material,but cannot be applied to an extruded product.

An object of several aspects of the invention is to provide an Al—Mg—Sialuminum alloy extruded product that exhibits high extrusionproductivity, high fatigue strength, excellent impact fractureresistance, and excellent formability.

SUMMARY

According to one aspect of the invention, there is provided an aluminumalloy extruded product that exhibits excellent fatigue strength andimpact fracture resistance, the aluminum alloy extruded productcomprising 0.3 to 0.8 mass % of Mg, 0.5 to 1.2 mass % of Si, 0.3 mass %or more of excess Si with respect to the Mg₂Si stoichiometriccomposition, 0.05 to 0.4 mass % of Cu, 0.2 to 0.4 mass % of Mn, 0.1 to0.3 mass % of Cr, 0.2 mass % or less of Fe, 0.2 mass % or less of Zr,and 0.005 to 0.1 mass % of Ti, with the balance being aluminum andunavoidable impurities, the aluminum alloy extruded product having afatigue strength of 140 MPa or more, a fatigue ratio of 0.45 or more,and an interval between striations on a fatigue fracture surface of 5.0μm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the compositions of aluminum alloys used for evaluation.

FIG. 2 shows evaluation results for billets or extruded products thatdiffer in alloy composition.

FIG. 3 shows property values and the like of extruded products subjectedto a solution treatment (immediately after extrusion).

FIGS. 4A and 4B show photographs used to evaluate the length ofcrystallized products.

FIGS. 5A and 5B show photographs used to evaluate striation.

FIGS. 6A and 6B show photographs used to evaluate a grain size.

FIGS. 7A to 7D show an example of a bending test (evaluation method)conducted on an extruded product and evaluation results.

FIGS. 8A and 8B show photographs used to evaluate orange peeling on abent surface of an extruded product.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

According to one embodiment of the invention, there is provided analuminum alloy extruded product that exhibits excellent fatigue strengthand impact fracture resistance, the aluminum alloy extruded productcomprising 0.3 to 0.8 mass % of Mg, 0.5 to 1.2 mass % of Si, 0.3 mass %or more of excess Si with respect to the Mg₂Si stoichiometriccomposition, 0.05 to 0.4 mass % of Cu, 0.2 to 0.4 mass % of Mn, 0.1 to0.3 mass % of Cr, 0.2 mass % or less of Fe, 0.2 mass % or less of Zr,and 0.005 to 0.1 mass % of Ti, with the balance being aluminum andunavoidable impurities, the aluminum alloy extruded product having afatigue strength of 140 MPa or more, a fatigue ratio of 0.45 or more,and an interval between striations on a fatigue fracture surface of 5.0μm or less.

The aluminum alloy extruded product according to one aspect of theinvention is characterized in that the Mg content and the Si content areset so that the aluminum alloy extruded product includes 0.5 to 1.5 mass% of Mg₂Si and 0.3 mass % or more of excess Si with respect to the Mg₂Sistoichiometric composition.

The term “fatigue ratio” refers to the ratio of the rotating fatiguestrength σ_(W) (10⁷ times) to the tensile strength σ_(B). The term“striation” refers to a line or a groove that forms a stripy patternthat occurs on a metal fatigue fracture surface due to slip planeseparation.

It is effective to reduce the maximum length of Al—Mg—Si crystallizedproducts to 10.0 μm or less in order to adjust the fatigue ratio to 0.45or more and the average interval between striations to 5.0 μm or less.

The maximum length of Al—Mg—Si crystallized products of an aluminumalloy ingot may be reduced to 10.0 μm or less by casting the ingot(cylindrical billet) at a casting speed of 80 mm/min or more (coolingrate: 15° C./sec or more).

Since such an aluminum alloy ingot exhibits excellent extrudability, theforming load (i.e., the stem pressure of an extrusion press machine)during extrusion can be set to be 0.9 or less with respect to an alloydefined in JIS 6061.

When producing the extruded product, it is preferable to reduce theaverage grain size of the extruded product to 50 μm or less.

The extruded product according to the invention exhibits excellent pressworkability and bendability. It is preferable that the extruded productsubjected to a solution treatment have an r-value (Lankford value) of0.7 or more or an n-value (work hardening exponent) of 0.23 or more ordoes not produce cracks on its surface when subjected to a bending testthat causes an outer surface elongation of 60% or more.

The content range of each component is described below.

Mg and Si

Si is necessary to maintain the strength of the aluminum alloy. However,the extrudability of the aluminum alloy is impaired if the Si content istoo high.

Mg is necessary to maintain the strength of the aluminum alloy. However,the extrudability of the aluminum alloy is impaired if the Mg content istoo high.

Therefore, the Mg content is set to 0.3 to 0.8 mass %, and the Sicontent is set to 0.5 to 1.2 mass %.

It is preferable to control the Mg₂Si content to 0.5 to 1.5 mass % andthe content of excess Si with respect to the Mg₂Si stoichiometriccomposition to 0.3 mass % or more taking account of precipitationhardening due to Mg₂Si.

The Si content and the Mg content significantly affect the mechanicalproperties (e.g., tensile strength and fatigue strength) of the aluminumalloy. When a fatigue strength of 160 MPa or more is required, it ispreferable that the Mg content be 0.45 to 0.8 mass %, the Si content be0.7 to 1.2 mass %, the Mg₂Si content be 0.7 to 1.5 mass %, and theexcess Si content be 0.45 mass % or more.

When a fatigue strength of 180 MPa or more is required, it is preferablethat the Mg content be 0.55 to 0.8 mass %, the Si content be 0.9 to 1.2mass %, the Mg₂Si content be 0.9 to 1.5 mass %, and the excess Sicontent be 0.6 mass % or more.

Cu

Cu improves the strength and the elongation of the aluminum alloy.However, the corrosion resistance and the extrusion productivity of thealuminum alloy deteriorate if the Cu content is too high. Therefore, theCu content is set to 0.05 to 0.4 mass %, and preferably 0.2 to 0.4 mass%.

Fe

Fe forms a crystallized product with Si if the Fe content is too high.As a result, the strength and the corrosion resistance of the aluminumalloy decrease. Therefore, the Fe content is set to 0.20 mass % or less,preferably 0.10 mass % or less, and more preferably 0.05 mass % or less.

Mn

Mn suppresses recrystallization to refine the grains of the aluminumalloy, and stabilizes the fiber texture of the aluminum alloy to improveimpact resistance. However, the quench sensitivity of the aluminum alloyincreases if the Mn content is too high so that the strength of thealuminum alloy decreases. Therefore, the Mn content is set to 0.2 to 0.4mass %, and preferably 0.3 to 0.4 mass %.

Cr

Cr suppresses recrystallization to refine the grains of the aluminumalloy, and stabilizes the fiber texture of the aluminum alloy to improveimpact resistance. However, the quench sensitivity of the aluminum alloyincreases if the Cr content is too high so that the strength of thealuminum alloy decreases. Therefore, the Cr content is set to 0.1 to 0.3mass %, and preferably 0.15 to 0.25 mass %.

Zr

Zr suppresses recrystallization to refine the grains of the aluminumalloy, and stabilizes the fiber texture of the aluminum alloy to improveimpact resistance. However, the quench sensitivity of the aluminum alloyincreases if the Zr content is too high so that the strength of thealuminum alloy decreases. Therefore, the Zr content is set to 0.20 mass% or less, and preferably 0.10 mass % or less.

Ti

Ti refines the grains of the aluminum alloy during casting. However, anumber of coarse intermetallic compounds are produced if the Ti contentis too high so that the strength of the aluminum alloy decreases.Therefore, the Ti content is set to 0.005 to 0.1 mass %.

Unavoidable Impurities

Unavoidable impurities do not affect the properties of the aluminumalloy if the content of each impurity element is 0.05 mass % or less andthe total content of impurity elements is 0.15 mass % or less.

Production Method

(1) A cylindrical billet is cast at a casting speed of 70 mm/min ormore, and preferably 80 mm/min or more (cooling rate: 15° C./sec) tocontrol the form of crystallized products.

(2) The billet is homogenized at 565 to 595° C. for four hours or more.

(3) The billet heating temperature during extrusion is set at 470° C. ormore so that the aluminum alloy extruded product is quenched. The upperlimit of the billet heating temperature during extrusion is about 580°C. or less taking account of local melting of the billet.

The cooling rate after extrusion is set at 500° C./min or more so thatthe aluminum alloy extruded product is quenched.

An artificial aging treatment is performed after quenching at 175 to195° C. for 1 to 24 hours (under-aging conditions).

According to one aspect of the invention, since the Al—Mg—Si aluminumalloy has the composition defined in claim 1 and has an average intervalbetween striations of 5.0 μm or less, high fatigue strength andexcellent impact fracture resistance can be obtained. Therefore, thealuminum alloy can be widely applied to a structural material (e.g.,automotive component) that is repeatedly subjected to impact duringtravel.

Since the extruded product has an r-value and an n-value equal to orlarger than given values, the extruded product exhibits excellent pressworkability and bendability.

Examples according to the invention are described below based oncomparison with comparative examples.

A molten aluminum alloy containing components shown in FIG. 1 (balance:aluminum) was prepared, and was cast at a casting speed shown in FIG. 1to obtain a cylindrical billet.

The billet was extruded into a round bar extruded product (diameter: 26mm) using an extruder. The extruded product was water-cooled immediatelyafter extrusion at a cooling rate of 500° C./min or more (die-endquenching), followed by artificial aging. FIG. 2 shows the propertyevaluation results.

FIG. 3 shows the evaluation results of the extruded product immediatelyafter extrusion (before artificial aging).

The properties of the extruded product were evaluated under thefollowing conditions.

Length of Crystallized Product

A specimen prepared from the center of the billet was etched (0.5% HF).The metal structure was observed using an optical microscope at amagnification of 1000 (measurement area: 0.166 mm², the maximum lengthof crystallized products was determined by image processing based on tenareas).

Striation

The metal structure at the center of the fracture surface of theextruded product that had been subjected to artificial aging and arotating bending fatigue test was observed using a scanning electronmicroscope at a magnification of 200 or 2000. In this embodiment, thenumber of striations was measured at intervals of 10 mm to calculate theaverage interval between striations.

Fatigue Properties

A JIS No. 1 (1-8) specimen (for rotating bending fatigue test) wasprepared from the extruded product subjected to artificial aging inaccordance with JIS Z 2274. The specimen was subjected to a fatigue testusing an Ono-type rotating bending fatigue tester conforming to the JISstandard.

Fatigue ratio=σ_(w)(10⁷ fatigue strength)/σ_(B)(tensile strength)

Tensile Properties

A JIS No. 4 tensile test specimen was prepared from the extruded productin accordance with JIS Z 2241. The specimen was subjected to a tensiletest using a tensile tester conforming to the JIS standard.

FIG. 2 shows the measurement results of the extruded product subjectedto artificial aging, and FIG. 3 shows the measurement results of theextruded product before artificial aging.

Impact Resistance

A JIS V-notch No. 4 specimen was prepared from the extruded productsubjected to artificial aging in accordance with JIS Z 2242. Thespecimen was subjected to a Charpy impact test using a Charpy impacttester conforming to the JIS standard.

Grain Size

A test material was mirror-polished and etched (3% NaOH, 40° C.×3 min).The metal structure of the test material was then observed using anoptical microscope at a magnification of 50 or 400.

Extrudability

The stem pressure of a press machine during extrusion was evaluated asextrudability (JIS 6061 alloy=1).

Bendability and Surface Properties

Bendability and surface properties shown in FIG. 3 were evaluated asfollows. Specifically, a specimen (20×150 mm) was prepared from theextruded product (test material) that had been water-cooled immediatelyafter extrusion and subjected to a solution treatment. As shown in FIG.7A, a test material 1 was placed on a lower jig 2, and a load wasapplied to the test material 1 from above using a punch 3 (R: 1.5 mm).

FIG. 7B shows a displacement-load diagram during the evaluation. FIGS.7C and 7D show examples of evaluation of the presence or absence ofcracks in the bent portion.

In FIGS. 7B to 7D, (A) indicates an example of an alloy of the exampleaccording to the invention (example extruded product), and (B) indicatesan example of an alloy of the comparative example (comparative extrudedproduct).

As shown in FIG. 7B, cracks did not occur in the extruded product (A) ofthe example according to the invention and showed a load displacementwith toughness. On the other hand, cracks occurred in the extrudedproduct (B) of the comparative example so that the load suddenlydecreased.

FIGS. 8A and 8B show photographs showing the surface properties of theextruded product (A) of the example according to the invention and theextruded product (B) of the comparative example after the bending test.

A case where only a small degree of orange peeling that did not affectthe fatigue strength was observed was evaluated as “Good”, and a casewhere significant orange peeling was observed was evaluated as “Bad”.

Note that the bent surface is normally elongated by 67% under the abovebending test conditions.

n-Value

A JIS No. 4 tensile test specimen was prepared from the extruded productthat had been water-cooled immediately after extrusion and subjected toa solution treatment in accordance with JIS Z 2241. The specimen wassubjected to a tensile test using a tensile tester conforming to the JISstandard. The n-value (i.e., an exponent n when a true stress-truestrain curve determined by a load-elongation curve is approximatelyindicated by σ=Fε^(n)) was calculated from the slope when the truestress-true strain value was plotted into the double logarithmic graph.

The n-value is referred to as a work hardening exponent. A large n-valueindicates excellent formability.

r-Value

A JIS No. 4 tensile test specimen was prepared from the extruded productthat had been water-cooled immediately after extrusion and subjected toa solution treatment in accordance with JIS Z 2241. The specimen wassubjected to a tensile test using a tensile tester conforming to the JISstandard. The ratio of the true strain in the widthwise direction to thetrue strain in the thickness direction of the specimen during thetensile test was calculated as the r-value (Lankford value).

Specifically, the width W₀ and the thickness T₀ of the specimen beforethe tensile test and the width W₁ and the thickness T₁ of the specimenafter the tensile test were measured, and the r-value was calculated bythe expression “r=(ln W₀/W₁)/(ln T₀/T₁)”.

A cooling rate of 15° C./sec or more was obtained for alloys No. 1 toNo. 5 (examples) shown in FIGS. 1 to 3 by setting the casting speed at80 mm/min or more.

A specimen was prepared from the center of the cylindrical billet, andthe metal structure was observed using an optical microscope afteretching the specimen. FIGS. 4A and 4B show photographs of the metalstructure.

The maximum length of Al—Fe—Si crystallized products (measured for tenareas, 0.166 mm²) of an alloy No. 2 (example) shown in FIG. 4A was 1.5μm (i.e., 10 μm or less). On the other hand, the maximum length ofAl—Fe—Si crystallized products of an alloy No. 13 (comparative example)shown in FIG. 4B was 12 μm.

FIGS. 5A and 5B show photographs of the center of the fracture surfaceof the extruded product that had been subjected to artificial aging andthe rotating bending fatigue test (10⁷ times).

The average interval between striations (measured at intervals of 10 mm)of the alloy No. 2 (example) shown in FIG. 5A was 0.5 μm (i.e., 5.0 μmor less). On the other hand, the average interval between striations ofan alloy No. 12 (comparative example) shown in FIG. 5B was 10.5 μm.

FIGS. 6A and 6B show photographs of the metal structure of the extrudedproduct.

The alloys of the examples according to the invention had an averagegrain size of 40 μm or less (i.e., 50 μm or less (target value)) (seeFIGS. 2 and 6A). On the other hand, alloys No. 11 and No. 12(comparative examples) had an average grain size as large as 400 to 800μm (see FIGS. 2 and 6B).

It is considered that the alloy No. 13 (comparative example) had anaverage grain size of 40 μm due to the effects of grain refinementcomponents (e.g., Mn and Cr). However, the length of crystallizedproducts in the billet was as large as 12 μm (see FIG. 2). As a result,the fatigue ratio (target value: 0.45 or more) and the impact value(target value: 60 J/cm²) did not reach the target values.

An alloy No. 10 (comparative example) that satisfied the target valuesshown in FIG. 2 had an Mg₂Si content of 1.53 mass % (i.e., outside therange of 0.5 to 1.5 mass %) and an excess Si content (“exSi” in FIG. 1)of 0.06 mass % (i.e., 0.3 mass % or less). As a result, the alloy No. 10exhibited an extrudability (indicated by the forming load duringextrusion) of 1.0 (target value: 0.9 or less) (see FIG. 3).

In the examples according to the invention, a fatigue strength of 140MPa or more and an impact value of 60 J/cm² or more were set as targetvalues on the assumption that the extruded product is applied to astructural material for which high fatigue strength and excellent impactfracture resistance are required.

As is clear from the results shown in FIGS. 2 and 3, when the length ofcrystallized products in the billet was 10.0 μm or less and the intervalbetween striations on the fatigue fracture surface was 5.0 μm or less,the forming load during extrusion was 0.9 or less with respect to analloy defined in JIS 6061. When the grain size of the extruded productwas 50 μm or less, the extruded product exhibited high fatigue strengthand had a high Charpy impact value.

In Examples 2-1 and 2-2 in which the Mg content was 0.55 to 0.8 mass %,the Si content was 0.9 to 1.2 mass %, the Mg₂Si content was 0.9 to 1.5mass %, and the excess Si content was 0.6 mass % or more, a fatiguestrength of 180 MPa or more and a proof stress of 370 MPa (i.e., higherthan those achieved in Examples 1 to 5) were obtained.

In Examples 2-1 and 2-2, although the Si content was set to be close tothe upper limit, the interval between striations was as small as 1.0 μmand the fatigue ratio was as high as 0.46 as a result of setting theexcess Si content to 0.6 mass % or more. Moreover, a high impact valueof 70 J/cm² or more (excellent impact fracture resistance) was obtained.

FIG. 3 shows the formability evaluation results of the extruded productsof the examples according to the invention and the extruded products ofthe comparative examples.

When producing automotive underbody parts or the like, an aluminum alloythat has been subjected to a solution treatment is generally subjectedto press working or bending before subjecting the aluminum alloy toartificial aging. Therefore, the target n-value and the target r-valueshown in FIG. 3 that indicate formability are set to 0.23 or more and0.7 or more, respectively.

The aluminum alloy extruded products of the examples according to theinvention achieved all of the target values, and did not produce cracksduring the 60% elongation bending test.

Although only some embodiments of the present invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the embodimentswithout materially departing from the novel teachings and advantages ofthis invention. Accordingly, all such modifications are intended to beincluded within scope of this invention.

1. An aluminum alloy extruded product that exhibits excellent fatiguestrength and impact fracture resistance, the aluminum alloy extrudedproduct comprising 0.3 to 0.8 mass % of Mg, 0.5 to 1.2 mass % of Si, 0.3mass % or more of excess Si with respect to the Mg₂Si stoichiometriccomposition, 0.05 to 0.4 mass % of Cu, 0.2 to 0.4 mass % of Mn, 0.1 to0.3 mass % of Cr, 0.2 mass % or less of Fe, 0.2 mass % or less of Zr,and 0.005 to 0.1 mass % of Ti, with the balance being aluminum andunavoidable impurities, the aluminum alloy extruded product having afatigue strength of 140 MPa or more, a fatigue ratio of 0.45 or more,and an interval between striations on a fatigue fracture surface of 5.0μm or less.
 2. The aluminum alloy extruded product as defined in claim1, the aluminum alloy extruded product being produced by extruding analuminum alloy ingot that has a maximum length of Al—Fe—Si crystallizedproducts of 10 μm or less.
 3. The aluminum alloy extruded product asdefined in claim 1, the aluminum alloy extruded product having anaverage grain size of 50 μm or less.
 4. The aluminum alloy extrudedproduct as defined in claim 1, a forming load during extrusion of thealuminum alloy extruded product being 0.9 or less with respect to analloy defined in JIS
 6061. 5. The aluminum alloy extruded product asdefined in claim 1, the aluminum alloy extruded product that has beensubjected to a solution treatment having a Lankford value of 0.7 ormore.
 6. The aluminum alloy extruded product as defined in claim 1, thealuminum alloy extruded product that has been subjected to a solutiontreatment having a work hardening exponent of 0.23 or more.
 7. Thealuminum alloy extruded product as defined in claim 1, the aluminumalloy extruded product that has been subjected to a solution treatmentnot producing cracks on its surface when subjected to a bending testthat causes an outer surface elongation of 60% or more.